Mind the (Seismic) Gap

San Andreas Fault in California.

Image via Wikipedia

The Seismic Gap Theory in geology has been somewhat controversial, even though some of its predictions appear to be rational, ie that movement of a tectonic fault along a plate boundary should be more or less continuous unless that slippage is blocked, in which case it is liable to sudden slippage (large earthquake) if strain starts to build.

The seismic gap hypothesis states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries. One of the earliest and clearest applications of the seismic gap theory to earthquake forecasting was by McCann et al. (1979), who postulated zones of high, medium, and low seismic potential around the Pacific rim.

A paper by Kagan and Jackson (1991) “Seismic Gap Hypothesis: Ten Years After” appeared to find that the Seismic Gap Hypothesis did not match reality:

In the 10 years since [1979], there have been over 40 large (M ≥ 7.0) earthquakes, enough to test statistically the earlier forecast. We also analyze another forecast of long-term earthquake risk, that by Kelleher et al. (1973). The hypothesis of increased earthquake potential after a long quiet period can be rejected with a large confidence. The data suggest that, contrary to these forecasts, places of recent earthquake activity have larger than usual seismic hazard, whereas the segments of the circum-Pacific belt with no large earthquakes in recent decades have remained relatively quiet. The “clustering” of earthquake times does not contradict the plate tectonic model, which constrains only the long-term average slip rate, not the regularity of earthquakes.

But in certain places, the long term slippage of a tectonic fault is more or less continuous in terms of low level earthquakes, unless for some reason, the slippage is impeded and the strain on the fault builds until a catastrophic earthquake occurs. This appears to have happened in the recent Christchurch (Lyttleton) earthquakes.

From Wolfram Alpha, the earthquakes above magnitude 3 for the last 30 years show a continuous range of small earthquakes and then a sudden year-long gap of no earthquakes at all followed by several large earthquakes that devastated Christchurch and the surrounding area.

Earthquakes near Christchurch, NZ in the last 30 years

The gap before the Lyttleton earthquake was noted by Warwick Hughes, but his graph was a little ideosyncratic.

The more recent series of devastating earthquakes off the east coast of Japan also show a short gap of a few months before the big ones happened:

Earthquakes for the last five years near Sendai, Japan

Clearly the Seismic Gap Hypothesis has some life left in it, after all.

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One Response to Mind the (Seismic) Gap

  1. terry says:

    Seismic Gap theory really is to be used for a singular stretch of a fault over a very long time period between ruptures.

    Like in the Himalayas, where there are several gaps, or along the Pacific Coast of South America where there are a number of gaps (including one that wasn’t quite filled by last year’s Chile quake), and so on. I think the theory itself is not quite debunked—I see references to it in many papers written, and in many areas of the world, it’s still used for hazard planning (Turkey, where one last “gap” remains near Istanbul, Japan which spent a lot of time and money preparing for the Tokai Big One and sort of dropped the ball on the Tohoku Big One, and elsewhere).

    These little gaps your data indicate are interesting, but they’re not quite, as I understand it, the seismic gaps as the theory describes.

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